In general, permanent magnet electric motors are broadly classified into two categories, that is, “surface permanent magnet motor” in which a permanent magnet is attached on the outer part of a rotor core, and “interior permanent magnet motor” in which a permanent magnet is embedded in a rotor core. Among these, the “interior permanent magnet motor” is suitable for use as a variable speed drive motor.
In a permanent magnet electric motor, the flux linkage of a permanent magnet is constantly generated at a uniform strength, as a result, an induced voltage due to the permanent magnet increases in proportion to the rotational speed. Consequently, during a variable speed operation from a low speed to a high speed, a high speed rotation causes a very high induced voltage (counter electromotive voltage) due to the permanent magnet. If the induced voltage due to the permanent magnet is applied to an electronic component of an inverter and increases up to equal to or more than the withstand voltage thereof, the electronic component results in dielectric breakdown. For avoiding such a result, a design is considered, in which the magnetic flux of a permanent magnet is reduced such that an induced voltage due to a permanent magnet is equal to or less than the withstand voltage of an electronic component of an inverter, but this design causes a reduction of output and efficiency in a low speed range of the permanent magnet electric motor.
During a variable speed operation approximating to constant output from a low speed to a high speed, the flux linkage of a permanent magnet is generated uniformly, in a high speed rotation range, the voltage of the motor reaches the upper limit of the power supply voltage and the current required for output is not to flow. As a result, in the high speed rotation range, the output is reduced significantly, and furthermore, a variable speed operation can not be performed over a wide range up to a high speed rotation
Recently, “flux weakening control”, which is described in various documents, has come to be applied to a method for enlarging the variable speed range. The total flux linkage quantity of an armature coil is determined from the magnetic flux due to a d-axis current and the magnetic flux due to a permanent magnet thereof. In the “flux weakening control”, a magnetic flux is generated by a minus d-axis current, and this flux due to the minus d-axis current causes reduction of the total flux linkage quantity. Further, in the “flux weakening control”, a permanent magnet with a high coercivity is adopted such that a working point of a magnetic characteristic (B—H characteristic) may change in the reversible range. Consequently, an NdFeB magnet with a high coercivity is applied to that of a permanent magnet electric motor such that the permanent magnet is irreversibly demagnetized by a demagnetizing field under the “flux weakening control”.
In operations applying the “flux weakening control”, the flux due to the minus d-axis current causes reduction of the flux linkage quantity, as a result, the reduced quantity of flux linkage generates a voltage to spare in relation to the upper limit voltage. An electric current to be a torque component increases, as a result, the output of a high speed range increases. In addition, the rotational speed increases in proportion to the voltage to spare, and this enlarges the range of the variable speed operation.
However, since a minus d-axis current does not contribute to the output, causing the minus d-axis current to flow constantly results in that the copper loss increases and the efficiency is deteriorated. Further, the demagnetizing field due to the minus d-axis current generates a harmonic flux, and increase of voltage due to the harmonic flux or the like makes a limit of voltage reduction due to the “flux weakening control”. For these reasons, even if “flux weakening control” is applied to an “interior permanent magnet motor”, it is impossible to perform a variable speed operation more than three times of a base speed. Further, there is a problem that the harmonic flux as described above causes an increase of iron loss and a substantial reduction of efficiency in a low and middle speed range. In addition, there is a possibility of generating a vibration as a result of electromagnetic force due to the harmonic flux.
When an “interior permanent magnet motor” is applied to a motor for driving a hybrid car, the motor is forced to operate in a state of engine drive only. In a middle and high speed rotation, an induced voltage of the motor due to a permanent magnet rises, for restraining the induced voltage to be equal to or less than the power supply voltage, a minus d-axis current is allowed to flow constantly by the “flux weakening control”. In this state, the motor generates only loss, as a result, an overall operating efficiency deteriorates.
When an “interior permanent magnet motor” is applied to a motor for driving an electric train, since the electric train runs by inertia at a state, as with the application to a motor for driving a hybrid car as described above, for restricting an induced voltage of the motor due to a permanent magnet to be equal to or less than the power supply voltage, a minus d-axis current is allowed to flow constantly by the “flux weakening control”. In this case, the motor generates only loss, as a result, an overall operating efficiency deteriorates.
As a technique for solving these problems, in Patent Document 1 and Patent Document 2, a technique is described, which arranges permanent magnets (called “variable magnetic force magnet” below) with a low coercivity such as to change magnetic flux density irreversibly due to a magnetic field generated by a stator coil, and permanent magnets (called “fixed magnetic force magnet” below) with a high coercivity that is equal to or more than twice as much as that of the variable magnetic force magnet, and adjusts the total flux linkage quantity by magnetizing the variable magnetic force magnet by magnetic fields due to current such that the total flux linkage due to the variable magnetic force magnet and the fixed magnetic force magnet is reduced in a high speed rotation range in which the voltage is equal to or more than the maximum voltage of the power supply voltage.
A permanent magnet electric motor of Patent Document 1, has a rotor 1 that is configured as shown in FIG. 15. Here, the rotor 1 is configured by a rotor core 2, eight variable magnetic force magnets 3 and eight fixed magnetic force magnets 4. The rotor core 2 is configured by layering silicon steel plates, the variable magnetic force magnets 3 are Al—Ni—Co magnets or FeCrCo magnets, and the fixed magnetic force magnets 4 are NdFeB magnets.
The variable magnetic force magnets 3 are embedded in the rotor core 2, and first type of cavities 5 each are provided at the opposite ends of the variable magnetic force magnets 3. The variable magnetic force magnets 3 each are arranged along with the radial direction of the rotor that matches the q-axis as a center axis between magnetic poles, and magnetized in the orthogonal direction to the radial direction. The fixed magnetic force magnets 4 are embedded in the rotor core 2, and second type of cavities 6 each are provided at the opposite ends of the fixed magnetic force magnets 4. The fixed magnetic force magnets 4 each are arranged approximately along with the round of the rotor 1 such that the fixed magnetic force magnets 4 each are put between two variable magnetic force magnets 3 on the inner side of the rotor 1. The fixed magnetic force magnets 4 each are magnetized in the orthogonal direction to the round of the rotor 1.
The magnetic pole parts 7 of the rotor core 2 each are formed such that they are surrounded by two variable magnetic force magnets 3 and one fixed magnetic force magnet 4. The center axis of the magnetic poles 7 is d-axis, and the center axis between each two magnetic poles 7 is the q-axis. In the permanent magnet electric motor of Patent Document 1 which adopts this rotor 1, a pulse-like current is allowed to flow through the stator coil in an extremely short conductive time (about 100 μs to 1 ms) to form a magnetic field, and thereby the magnetic field acts on the variable magnetic force magnet 3. When a magnetizing magnetic field is 250 kA/m, ideally, sufficient magnetizing magnetic field acts on the variable magnetic force magnet 3, and the fixed magnetic force magnet 4 is not irreversibly demagnetized due to the magnetization.
As a result of this, with the permanent magnet electric motor of Patent Document 1, the flux linkage of the variable magnetic force magnets 3 changes from the maximum value to zero, and magnetization can be made in the two directions of forward and reverse. In other words, if the flux linkage of the fixed magnetic force magnets 4 is directed in a forward direction, the flux linkage of the variable magnetic force magnets 3 can be adjusted in a range of from the maximum value of the forward direction to zero, and furthermore, over a wide range up to the maximum value of the reverse direction. Consequently, in the rotor 1 of Patent Document 1, the variable magnetic force magnets 3 are magnetized by a d-axis current, as a result, the total flux linkage quantity can be adjusted over a wide range, which is obtained by summing up that of the variable magnetic force magnets 3 and that of the fixed magnetic force magnets 4.
For example, in a low speed range, when the variable magnetic force magnets 3 are magnetized by a d-axis current such that the flux linkage of the variable magnetic force magnets 3 shows the maximum value in the same direction (in a initial state) of the flux linkage of the fixed magnetic force magnets 4, the torque due to the permanent magnets reaches the maximum value, and this allows the torque and output of the motor to be the maximum. In a middle and high speed range, when the magnetic flux is reduced and the total flux linkage quantity is reduced, the voltage of the motor is reduced, and this generates a voltage to spare in relation to the upper limit of the power supply voltage, and allows the rotation speed (frequency) to be higher.    Patent Document 1: Japanese Patent Application Laid-open No. 2006-280195    Patent Document 2: Japanese Patent Application Laid-open No. 2008-48514    Non-patent Document: “Design and control of embedded magnet synchronous motor”, Yoji Takeda et al., Ohmsha, Ltd., October 2001, ISBN: 4-274-03567-0
The permanent magnet electric motor of Patent Document 1 which is configured as described above, has an excellent characteristics that the flux linkage quantity of the variable magnetic force magnets 3 changes over a wide range of from the maximum value to zero by a d-axis current of the rotor 1, and that and magnetization can be made in the two directions of forward and reverse. On the other hand, a large magnetization current is required when magnetizing the variable magnetic force magnets 3, this allows an inverter for driving the motor to be enlarged.
In particular, in view of characteristics of permanent magnets, a large magnetization current is required when magnetization compared with when demagnetization. However, since the permanent magnet electric motor of Patent Document 1 has a configuration in which two types of magnets are arranged magnetically in parallel, by the influence of the flux linkage of the fixed magnetic force magnets 4, a large magnetic field is required for magnetization of the variable magnetic force magnets 3.
FIGS. 16(A) to 16(D) are schematic diagrams illustrating this. In the permanent magnet electric motor of Patent Document 1, as shown in FIG. 16(A), two variable magnetic force magnets 3 and one fixed magnetic force magnet 4 are arranged in a U-shape such that d-axis is center of the U-shape. When the motor is operating in a normal state, the flux of the variable magnetic force magnets 3 and the fixed magnetic force magnet 4 is directed in the direction of the center magnetic pole part 7. In this state, when a d-axis current flows in pulse-like to generate a magnetic field for demagnetization, the flux of the magnetic field is generated such that the flux penetrates the variable magnetic force magnets 3 and the fixed magnetic force magnet 4 from the outer side of the rotor 1, as shown in FIG. 16(B), resulting in that the variable magnetic force magnets 3 are demagnetized. In this case, since the fixed magnetic force magnet 4 has a high coercivity, the magnet 4 is not demagnetized.
During the demagnetization process, as shown in FIG. 16(B), the flux of the fixed magnetic force magnet 4 flows in the d-axis direction and in the direction of going from the inner side to outer side of the variable magnetic force magnets 3, that is, in the reverse direction of an initial direction of the flux of the variable magnetic force magnets 3, thereby the fixed magnetic force magnet 4 assists the demagnetization due to a magnetic field generated by a d-axis current. Consequently, as shown in FIG. 16(c), the demagnetization can be made up to the reversal of the polarity of the variable magnetic force magnets 3.
On the other hand, in a magnetization process, a d-axis current is applied in pulse-like again, as shown in FIG. 16(D), a magnetic field is generated in the reverse direction to that of FIG. 16(B), the flux in the reverse direction which configures the magnetic field, restores the flux linkage of the demagnetized variable magnetic force magnets 3 to the state of a normal operation of FIG. 16(a). However, essentially, a large energy is required for the magnetization compared with the demagnetization, in addition to this, as shown in FIG. 16(D), the flux of the fixed magnetic force magnet 4 is applied to the variable magnetic force magnets 3 in the direction of demagnetizing the variable magnetic force magnets 3, as a result, a large magnetization currant is required, which can generate a large magnetic field to overcome this.
As described above, in the permanent magnet electric motor of Patent Document 1, since the two types of magnets are arranged magnetically in parallel, there are merits such as a large magnetization of the variable magnetic force magnets 3 and increase of the variation of the magnetic force such as from zero to 100%, but there is a demerit in which a large magnetization currant is required for a magnetization process.
The demerit as described above, is not limited to the magnetization process, and occurs not a little when demagnetizing the variable magnetic force magnets 3. What is desired is a permanent magnet electric motor which allows the magnetic flux of the variable magnetic force magnet 3 to be changed efficiently.
This invention is proposed to solve the problems described above, and has as an object the provision of a permanent magnet electric motor which can reduce a magnetization current when magnetizing variable magnetic force magnets, thereby which, without enlarging an inverter, can perform a variable speed operation over a wide range of from a low speed to a high speed, as a result, which can contribute to a high torque in a low speed rotation range, a high power output in a middle and high speed rotation range, and an improvement of efficiency.